Conventionally, a light or radiation image pickup apparatus has a light or radiation detector for detecting light or radiation. Light herein refers to infrared rays, visible light, ultraviolet rays, radiation, gamma rays and so on. X-rays in particular will be described by way of example. As an X-ray detector, a flat panel X-ray detector is in wide use which detects X-rays using an active matrix substrate. This is because the active matrix substrate is very useful in that X-ray detection values can be read on a pixel-by-pixel basis. Further, where an X-ray conversion layer consisting of a semiconductor is laminated on the active matrix substrate, an X-ray detecting element can be formed for each active element.
When a semiconductor layer is used as the X-ray conversion layer, X-rays incident on the X-ray conversion layer can be converted into charge signals (carriers). These converted charge signals are stored in capacitors provided for the respective X-ray detecting elements. The stored charge signals are read by the active matrix substrate for the respective X-ray detecting elements, and are further amplified while being converted from the charge signals into voltage signals. Based on these voltage signals, an image processor can construct an X-ray transmission image.
The voltage signals sent to the image processor in this way include, besides the voltage signals based on the charge signals converted from X-rays, noise signals which are voltage signals caused by dark current in the X-ray conversion layer, and voltage signals caused by amplifier noise generated when amplifying while converting from the charge signals into the voltage signals.
With amorphous selenium (α-Se) film conventionally employed as the X-ray conversion layer, the noise signals caused by dark current do not greatly change in response to temperature change. Since the amplifier noise does not change greatly in response to temperature change, either, dark image signals (hereinafter called offset signals) occurring when X-rays are not emitted are periodically measured, and these offset signals are removed as noise signals as in Patent Document 1, for example.